QPID-5036: View Message dialogue can omit or display stale field values.

* Fixed mismatched id between json and html (message-expiration vs message-expirationTime).
* Fixed display of stale values (values from previously viewed message would appear).
* Fixed display of fields with the boolean value false.
* MessageServlet changed so that JSON fields expirationTime and timestamp are included if they have non-zero values.
* MessageServlet fails to set content type when producing JSON for single message.  This caused a "not well-formed" error in Firefox Console.

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1510429 13f79535-47bb-0310-9956-ffa450edef68